Technical Abstract:
Medicago truncatula is a useful model for identifying genes of agronomic importance in legumes with more complex genomes. We are using microarray transcript profiling to identify genes involved in disease resistance responses and aluminum tolerance. A collection of accessions of M. truncatula was screened for reaction to two foliar pathogens, Colletotrichum trifolii and Erysiphe pisi, to identify resistant and susceptible germplasm. Epidermal cells of resistant genotypes responded to infection with a hypersensitive response, producing hydrogen peroxide and fluorescent phenolic compounds. The profile of up-regulated genes in the resistant interactions was consistent with the cellular responses observed and included flavonoid/lignin pathway genes, a xyloglucan-specific fungal endoglucanase inhibitor, a phospholipase A2, and a ubiquitin-protein ligase, among others. Seventeen unknown genes, including at least two that are legume-specific, were up-regulated in resistant genotypes. Aluminum (Al) toxicity is one of the causes of yield reduction in acid soils. Screening accessions in solution culture identified germplasm with greater tolerance than the genotype Jemalong A17. In one accession, tolerance is associated with lysigeny of Al-intoxicated epidermal root cells. Microarray profiling identified suites of genes expressed in tolerant and sensitive genotypes. The effect of silencing selected genes using RNAi in transgenic hairy roots was measured by Al uptake and root elongation. Decreased expression of a gene involved in pectin modification significantly reduced Al binding in hairy roots demonstrating the utility of a reverse genetics approach for increasing understanding of Al responses and improving tolerance in cultivated varieties.